1. Field of The Invention
The invention relates to a chip carrier socket for use with chip carriers which have leads thereon. The invention is particularly concerned with providing a low profile chip carrier socket where adequate contact forces of the contacts are achieved against leads on a chip carrier in the socket. The contact force is to be sufficient to compensate for the dimensional variations in the chip carrier as a result of manufacturing tolerances or designated specifications.
2. The Prior Art
Integrated circuits are commonly mounted in chip carriers which are bodies of insulating material having side surfaces from which conductors extend to form the integrated circuits. Traditionally, leads are provided on the side surfaces of the chip carrier body and contact is made with these leads to connect the integrated circuit to conductors on a substrate such as a ceramic substrate or a circuit board. Contact is usually established with the leads on the chip carrier by means of a chip carrier socket which comprises a socket body having in the center portion a recess which receives the chip carrier and has the contacts of the socket in surrounding relationship to the recess so that when the chip carrier is placed in the recess, the contacts of the socket contact the leads of the chip carrier.
Notwithstanding the relatively small size of the chip carrier socket and the contacts in the socket due to miniaturization trend, it is necessary that each contact be capable of exerting a substantial contact force on a respective lead of a chip carrier in order to establish good electrical contact between the contacts and leads on the chip carrier.
Another trend for electrical assembly packaging industry is Surface Mount Technology (SMT). Although traditional chip carrier sockets were mounted on a circuit board with the through-board contact tails by wave soldering process, it is preferred now that the sockets are designed to be surface mounted by using solder pastes printed on the board and in a later time heated through vapor phase process or infrared soldering process so the chip carrier socket is connected to the board by these reflow solder pastes.
For pursuing a better contact force, most manufacturers made a great effort in contact design in order to obtain a resilient contact having a zero insertion or low insertion force but having sufficient retention or contact force with the corresponding lead on the chip carrier, for example, U.S. Pat. Nos. 5,067,904, 5,009,609, 4,968,259, 4,941,832, 4,993,955, 4,872,845, 4,936,783, 4,547,031, 4,846,403, 4,886,470, 4,761,140, 4,919,623, 4,746,299, 4,984,991, 4,684,184, 4,846,704, 4,679,871, 4,691,975, and 4,547,031 . . . .
Generally speaking, the contact is designed to have a sufficiently free length at best so that the contact can own a good resilience when it engages the corresponding lead. In contrast, low profile is a trend in computer industry so it means less space and length can be used to house the contact therein. Accordingly, it is impossible to have the contact have a significant length in a vertical direction. As aforementioned, another trend is surface mount instead of wave soldering. In other words, there is no longer a downward extending tail to be adapted to be received within and through a hole of a board, so that the height of an SMT contact is less than that of a conventional contact having a extending tail. Therefore, based on limited dimension to be utilized, it is more difficult to design a SMT contact having sufficient resilience than a conventional wave soldering or through-hole contact.
Using a forming type contact is a effort which has been made. Forming is a manufacturing method whereby a sheet is cut into pieces as raw components. Each piece is lengthwise bent successively to form a continuous curve in a lateral direction as shown in U.S. Pats. Nos. 4,547,031, 4,679,871 and 4,761,140. The benefit of forming is that the final formed component has a better spring character because bending or forming has in part destroyed the inner structure of the corresponding component so that the stiffness about the bending portion is decreased and more flexibility occurs thereabout. While forming can offer a better spring character, disadvantageously forming is a complex and multiple process and it costs money and takes time. Also, there is manufacturing variations in these successively bending or forming process so it is not easy to control the final product having the designated shape and position, and owning the designated spring character. Additionally, because of tooling limitation, it is impossible to design a sharp angle or a tiny embossment on the surface of the contact during forming. This prohibits some optimal options in design and limits applicable space. Besides, a larger width of the forming component is generally required to ease forming to avoid tilting. In other words it is impossible to arrange a plurality of side by side contacts having a fine pitch within a socket, and no high density contact arrangement can be achieved. Another disadvantage of the forming contact is that because the contact is formed and bent by a single piece, it is difficult to make a protrusion portion which can extend to the outer surface of the socket wall for probe testing when the socket has been mounted on a board and some contact status is required to be a reference. In brief, forming is not a economic and simple method, and easily has variations.
In contrast, using a stamped contact is more popular now. In general, stamping is a easy way to produce the contact. Also, it is easy to control the position and dimension of the product, so no adjustment is required after stamping. Additionally, there is hardly any limitation for stamping to design a sharp angle or a tiny embossment on the surface of the product. Accordingly, the designer can almost utilize the whole space of the contact to achieve a best spring character thereof. Besides, it is unnecessary to use a thicker sheet to form the contact so that it saves the material, and fine pitch and high density contact arrangement can be more easily obtained than by formed contacts.
Although stamping has so many advantages, it still has a big problem for its stiffness. Unlike forming or bending which has damaged a portion of inner combination structure around the bending portion, any part of the stamped contact has the same property as the original sheet. In other words, the spring character is only provided by the shape itself, not by additional pre-bending operation. It is much tougher than the formed contact.
Because there are limitations of low profile and surface mount issues, to design a stamped contact used in those conditions gets more difficult than usual. Some effort has been made in U.S. Pat. No. 4,941,832 which uses stamped contacts in an SMT condition and tries to still achieve the sufficient resilient retention force. The means to operate the function in '832 is to use a mounting arm 32 which is bent inward to accommodate SMT type connection. Although it may achieve the resilience it wants, it still needs at least two-step bending process to form the final shape. Accordingly, there is still some applicable limitations for that contact when it is designed.
In according with this invention, a new design of contact has advantages over aforementioned prior art design. The present invention is specially directed to the specific shape of the contact which allows the contact to have a less height accommodating the low profile socket but still offering a sufficient resilient force to retain the corresponding lead of a chip carrier. Additionally, the contact is formed only by stamping to simplify the manufacturing process for lowering the cost, and also high density and fine pitch contact arrangement can be considerably designed when required. The contact of the present invention also has a benefit for easily using a probe to check the contact status after the socket has been mounted on the board.